A collaboration between China, Russia, India, Japan Korea, the US and the EU, ITER’s reactor will be larger and far more intricate than any previous tokamak. It will have as many as 10 million parts – its builders call it the puzzle with 10 million pieces – and will sit at the centre of a vast support system. The result will rival the Large Hadron Collider for the title of most complex machine on earth.

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Progress on ITER has been slow – it was first conceived during diplomatic talks between US president Ronald Reagan and Soviet leader Mikhail Gorbachev in 1985. Now, at last, the pieces of the puzzle are falling into place, although most of the ITER site, at Cadarache in southern France, is still barren. That is because the real action is taking place elsewhere.

Sun, sea and steel

The French Riviera is more generally associated with sun and sea than with mega engineering projects. When I visit the facility in La Seyne-sur-Mer where some of ITER’s biggest components are being prepared, a fierce mistral is blowing off the land to the Mediterranean. CNIM, the contractor that owns the facility, started out as a shipbuilder before turning to precision engineering. Its maritime location is an advantage&colon; many of ITER’s components are so heavy that they have to be transported by sea.

In one of the facility’s climate-controlled warehouses, a huge drill is carving channels into a D-shaped loop of high-grade stainless steel- so large that it takes me nearly a minute to walk its circumference. The steel, chosen for its strength at low temperatures, is so tough that the carbide bits milling it must be replaced every eight minutes. It needs to be&colon; seven of these loops will be stacked on top of each other to form one of the many magnets that will confine and direct hydrogen plasma at up to 100 million °C in the reactor vessel.

Before that, though, a complicated journey lies ahead. The loops’ next stop will be La Spezia, Italy, where a contractor will fit up to 700 metres of superconducting cable to each one; then they will travel to Venice, where another firm, Simic, will complete their assembly into structures called toroidal field coils, each weighing about the same as a fully laden Boeing 747. Simic is also milling some of the loops, so those will have to make a round trip to La Spezia and back.

The coils will then voyage to a French port, where they will be loaded onto a 800-tonne, 352-wheeled crawler that inches through 104 kilometres of countryside, crossing specially strengthened bridges and squeezing through carefully widened roads, to Cadarache. If all goes to plan, the first coils will arrive at the ITER site in about three years’ time.

Deadline implausible

Still, progress on the toroidal coils seems faster than on the second of the reactor’s key magnetic arrays, the so-called poloidal field coils. The building specially built for their construction is impressively large but mostly empty, save for half a dozen crates and a circular crane that hangs from the roof like a vast yellow spider, as it has been since 2012 when New Scientist last visited.

Following mounting criticism of ITER’s progress, director-general Osamu Motojima is striving to put the monumental project on “a more realistic schedule”. He told New Scientist that the difficulty of integrating the parts supplied by ITER’s myriad partners made the current deadline of 2020 for “first plasma” implausible; 2022 or 2023 are more likely.

Even once first plasma has been achieved, the reactor will spend years running experimentally before switching to the deuterium-tritium mix needed to generate substantial power. Motojima hopes this second milestone, scheduled for 2027, will still be achievable.

All this is taking its toll on morale. Several of the senior ITER figures I spoke to felt that ITER’s politics – with member states jostling for contracts, and supposedly identical parts often made by different manufacturers on different continents – together with the technical challenges, made even Motojima’s revised timeframe unworkable. They are quietly banking on 2025 or beyond. “I hope I see first plasma while I’m still on the project,” says Neil Mitchell, head of ITER’s magnets division.

Others are enjoying the ride. “ITER is not the first mega project&colon; it’s a great challenge, but it’s also great fun,” says Ken Blackler, who will have the job of fitting together the giant components inside the tightly confined wall of the reactor, Tetris-style.

The most impassioned advocate I hear from is Mark Henderson, who runs the microwave system that will help heat the plasma. He argues forcefully that fusion is the only adequate response to climate change. “Grasping the sun and bringing it to earth is greater than going to the moon and decoding DNA,” he says. But he too agrees that the rate of development needs to accelerate, and the road to practical fusion power may be a long one.

Those working on ITER today may have to live in the knowledge that the fruit of their labours will be reaped by others.

The writer’s accommodation, meals and travel within France were provided by Fusion For Energy

Article amended on 19 May 2014

When this article was first published, it reversed the direction of the mistral. This has now been corrected.